Interest Calculator

Use this free interest calculator to estimate simple interest or compound interest with principal, annual rate, time, compounding frequency, and monthly contributions.

All tools

Formula steps Estimate limits shown Examples included Private history

Principal ($)Interest rate (%)Time (years)

Simple interest$150.00

$1,000 x 5% x 3 years

Ending balance: $1,150.00
Principal: $1,000.00
Time: 3 years

Formula steps

Convert 5% to decimal rate 0.05.
Multiply principal by annual rate and years.
Add interest to principal for the ending balance.

How to use the interest calculator

Enter the requested dollar amounts, rates, terms, tax settings, or contribution details.
Use rates as percentages, such as 6.5 for 6.5%, and check whether a field asks for a monthly or annual amount.
Press the calculate button to see the answer, supporting metrics, and formula steps.
Use the result as a planning estimate only, then copy it if the assumptions look right.

Common uses

Compare simple interest with compound interest.

Estimate interest earned on savings or interest charged on a balance.

Test how contribution size and time change compound growth.

Build intuition before using the investment or compound interest calculators.

Examples

Simple interest $1,000 at 5% for 3 years

$150 interest before any fees or tax

Compound growth $2,500 at 6% for 10 years

Ending balance with compounding

Monthly deposits $1,000 plus $100/month at 6%

Contribution growth estimate

Frequently asked questions

Quick answers about formulas, rates, payment assumptions, estimate limits, examples, and private in-browser history.

What can I use the Interest Calculator for?

Use it for quick planning, comparison, and what-if estimates before you check exact numbers with a lender, tax professional, payroll provider, or financial adviser.

How does the Interest Calculator calculate the result?

Simple interest is principal times rate times time. Compound interest grows the balance by the effective periodic rate and can include monthly contributions.

Is this financial, tax, or legal advice?

This calculator gives an educational estimate only. It does not include every fee, lender rule, tax rule, local rate, credit, penalty, or personal financial detail.

Are my finance inputs private?

Yes. The calculator runs in your browser tab. Recent answers stay only on the page while you use it and are not sent to a server.